La-H化合物对LaMg2Ni合金中Mg2Ni相吸氢过程的影响

裴立超韩树民＊,朱惜林刘宝忠赵 鑫扈 琳

(1燕山大学亚稳材料制备技术与科学国家重点实验室，秦皇岛 066004)

(2燕山大学环境与化学工程学院，秦皇岛 066004)

La-H化合物对LaMg2Ni合金中Mg2Ni相吸氢过程的影响

裴立超1,2韩树民＊,1,2朱惜林2刘宝忠1赵 鑫2扈 琳2

(1燕山大学亚稳材料制备技术与科学国家重点实验室，秦皇岛 066004)

(2燕山大学环境与化学工程学院，秦皇岛 066004)

采用感应熔炼技术在Ar气氛保护下制备得到LaMg2Ni与Mg2Ni合金。X射线衍射(XRD)图表明LaMg2Ni合金在吸氢过程中分解为LaH3相和Mg2NiH4相，放氢过程中LaH3相转化为La3H7相。与Mg2Ni合金相比，LaMg2Ni合金显示出优良的吸氢动力学性能，这是由于镧氢化合物的存在及其在吸氢过程中所发生的相转变所造成的。LaMg2Ni合金280 s内吸氢即可达到最大储氢量的90%以上，而Mg2Ni合金则需要1200 s才能达到，且在相同温度下LaMg2Ni合金的吸氢反应速率常数大于Mg2Ni合金速率常数。镧氢化合物不仅有利于改善动力学性能，而且可以提高热力学性能。LaMg2Ni合金中的Mg2Ni相氢化反应焓与熵分别为-53.02 kJ·mol-1和 84.96 J·K-1·mol-1(H2)，这一数值小于单相 Mg2Ni氢化反应焓与熵(-64.50 kJ·mol-1,-123.10 J·K-1·mol-1(H2))。压力-组成-温度 (P-C-T)测试结果表明在603 K至523 K温度范围内，LaMg2Ni合金储氢容量保持稳定为1.95wt%左右，然而Mg2Ni合金的储氢容量则由4.09wt%衰减为3.13wt%，Mg2Ni合金的储氢容量在523K低温下仅为603 K时的76.5%，表明镧氢化合物能够改善Mg2Ni合金低温下的吸放氢性能。

储氢合金；相转变；镧氢化合物；吸氢动力学

A great deal of research on hydrogen as an alternative energy source has been carried out during the past decades,many hydrogen storage materials have been discovered for storing and transporting hydrogen safely and economically.Among these materials,Mg has been considered as a promising candidate because of its high hydrogen capacity(up to 7.60wt%),reversibility and low cost[1].However,its poor kinetic properties during hydrogenation/dehydrogenation (H/D)process and the normally required high dehydrogenation temperature (＞573 K),has limited its industrial application.

Untilnow,there are severalapproachesto improve the properties of Mg-based alloy.One of the most possible solutions is alloying with transition metal(Ni,Cu and Ti,etc.),such as Mg2Ni[2-3]alloy,to improve kinetic performances of pure magnesium.A further improvement of absorption/desorption conditions may be obtained by alloying with rare earths,the rare earth hydride corresponding to rare earth element forms in the hydrogen adsorption process,and the rare earth hydride can effectively catalyze hydriding/dehydriding kinetics[4-5].Other reported methods for the improvement of the Mg based materials hydrogen storage ability include ball milling with catalyst[6-7],surface modification with acid[8]and some new preparing methods for hydrogen storage alloys such as Hydriding Combustion Synthesis (HCS)[9],Spark Plasma Sintering[10]etc.

Recently,many researches[11-12]have focused on LaMg2Ni compound,an amorphous phase of LaMg2Ni could be obtained by means ofmeltspinning technique,hydrogenation at 443 K leads to the formation of LaMg2NiH7;at higher temperatures(523 K)LaHxphase and Mg2NiH4phase were produced,but the effect of La hydride compound was not mentioned in LaMg2Ni alloy[13].Ouyang et al.[14]found that the actual hydrogen absorption phase was Mg2Ni phase for the LaMg2Ni alloy prepared by inductive melting and LaH2.46phase existed in the whole process,the LaH2.46phase was helpful to improve hydriding kinetics of LaMg2Ni alloy.However,it did not explain in detail how the La hydride compound acted as a catalyst in LaMg2Ni alloy.Herein we report the hydrogen storage ability of LaMg2Ni alloy and its phase transition during the hydriding/dehydriding (H/D)process,in particular,effect of La hydride compound on hydriding process of Mg2Ni phase in LaMg2Ni alloy.

1 Experimental

LaMg2Ni and Mg2Ni ingots were prepared by inductive melting of high-purity La,Mg and Ni(purity more than 99.9%)in a magnesia crucible under argon atmosphere.A slight excess of Mg was used to compensate for evaporative Mg loss during the melting procedure[15].Then the ingots were annealed at 738 K for 6 h.The composition of these alloys was analyzed by Inductive Coupled Plasma Emission Spectrometer(ICP).The phase structures of the as-cast alloy and hydrogenated alloy were measured on a D/max-2500/PC X-ray diffractometer withCu Kα radiation (λ=0.154 06 nm).The X-ray intensity was measured at 40 kV,100 mA over a diffraction angle from 10°to 80°with a scan rate of 2°·min-1.The cell unite volume was calculated by Jade-5 software.The mechanical ball milling experiment was carried out by Pulverisette 6 planetary mono mill made in Germany;ball vs.sample ratio was 15∶1.The hydriding/dehydriding behaviorwasmeasured by Pressure-Composition-Temperature(P-C-T)characteristic measurement equipment (made by SuzukiShokan,Japan).The measurement conditions were set as:delay time 300 s,maximum pressure 3.0 MPa.The specimen weight for PC-T measurement is～ 2.0 g.The hydriding kinetic of the as-cast alloy was also tested by P-C-T characte-ristic measurement equipment under the initial hydrogen pressure of 3.0 MPa.The activation conditions could be illustrated as follows,the LaMg2Ni and Mg2Ni alloys hydrogenated under 3.0 MPa hydrogen pressure for 2 h,and the dehydriding process was in vacuum for 2 h,the temperature of activation was 623K.

2 Results and discussion

Fig.1 shows the XRD patterns of LaMg2Ni alloy after hydriding/dehydriding process.From pattern(a),it reveals that LaMg2Ni alloy transforms to LaH3phase and Mg2NiH4phase after hydriding process at 623 K,while there is no LaMg2NiH7phase peak,which is consistent with the results of references[16-17].The reaction of LaMg2Ni phase can be summarized as follows:

To understand phase transition during the hydriding/dehydriding (H/D)process more clearly,XRD pattern of LaMg2Ni alloy after dehydriding process at 623 K was collected and is shown in Fig.1(pattern (b)).Apparently,after dehydriding process,LaH3phase transforms to La3H7phase,while Mg2Ni phase exists,the reaction can be written as follows:

This phase transition during hydriding/dehydridng process was not found before,we presume that besides the existence of La hydride compound,this phase transition plays an important role in improving hydriding/dehydridng properties of LaMg2Ni alloy.

For improving hydriding/dehydriding kinetics of LaMg2Ni and Mg2Ni alloys,activation is made at 623 K and hydrogen absorption curves are shown in Fig.2 and Fig.3,the initial hydrogen pressure is 3.0 MPa.The as-cast Mg2Ni alloy can not be activated at 623 K,so activation curves of Mg2Ni alloy are measured after ball-milling for 2 h.It is clearly seen from Fig.2 that at the first activation cycle,the uptake time for 90%hydrogen content of the maximum hydrogen storage capacity of LaMg2Ni alloy is 1 560 s,for Mg2Ni alloy is 1510 s.For the second activation cycle,from Fig.3,the uptake time for 90%hydrogen content of the maximum hydrogen storage capacity of Mg2Ni alloy is 1200 s,while for LaMg2Ni alloy,it only needs 280 s.According to XRD analysis,in LaMg2Ni alloy,it decomposes to La hydride compound and Mg2NiH4during hydriding process,the existence of La hydride compound accelerates hydrogen absorption/desorption rate,because it increases reactive surface area greatly and decreases diffusion length of hydrogen[4,18].MoreoverLahydridecompound undergoesphase transition from La3H7phase to LaH3phase during hydriding process,the unit cell volume of La3H7phase is 0.357 4 nm3with tetragonal La3H7type structure and that of LaH3phase is 0.176 1 nm3with cubicCeH3type structure.The unite cell volume of La3H7is about two times of LaH3,although the whole volume of La hydride compound does notchange during hydriding process,lattice interface of LaMg2Ni alloy becomes larger than before,so we believe that the cocatalysis of existence of La hydride compound and this phase transition is beneficial to improve hydriding kinetics of LaMg2Ni alloy.

The hydrogen absorption process of LaMg2Ni and Mg2Ni alloys can be best fitted to Eq.(1)(Jander rate equation)

The Eq.(1)is rate expression of Jander Diffusion Model(JDM),where α is the reacted fraction vs.time t,k is the rate constant.

The hydrogen absorption process can be described by three-dimensional diffusion mechanism,and the temperature-dependent rate constants(k)are obtained from the slope of each of the straight lines obtained from Fig.4 and Fig.5.

The rate constants of LaMg2Ni and Mg2Ni alloys at different temperatures are shown in Table 1.The k value of LaMg2Ni alloy is larger than that of Mg2Ni alloy at the same temperature,which indicates that LaMg2Ni alloy shows better hydriding kinetics than Mg2Ni alloy.This result is in good agreement with the above analysis about activation for these two alloys.

Table 1 Rate constants of LaMg2Ni and Mg2Ni alloys at different temperature

Hydrogen storage performance of LaMg2Ni and Mg2Ni alloys are evaluated by measuring P-C-T at different temperatures.Fig.6 and Fig.7 show the P-C-T curves of the two alloys measured at 603 K,573 K and 523 K.It is worth noting that this set of data was collected right after the activation H/D cycle at 623 K and tested in order of the reduction of temperature.Asshown in Fig.6,hydrogen storage capacity of LaMg2Ni alloy is 1.98wt% (603 K),1.92wt% (573 K)and 1.91wt% (523 K),respectively.Its hydrogen storage capacity keeps stably with the temperature decreases from 603 K to 523 K.It is clearly seen from Fig.7 that hydrogen storage capacity of Mg2Ni alloy is 4.09wt%(603 K),3.63wt%(573 K)and 3.13wt%(523 K),with thereduction oftemperature,hydrogen storage capacity of Mg2Ni alloy declines distinctly.The relative hydrogen storage capacity of LaMg2Ni and Mg2Ni alloys vs.temperature is shown in Fig.8.The relative hydrogen storage capacity is defined and calculated by the following equation:

Where STis the relative hydrogen storage capacity at T temperature,CTis the hydrogen storage capacity at T temperature and C603Kis the hydrogen storage capacity at 603 K.

From Fig.8,it can be seen that the S523Kof LaMg2Ni alloy is 96.4%,while that for Mg2Ni alloy is 76.6%.The hydrogenation plateau of LaMg2Ni alloy is higher than Mg2Ni alloy at the same temperature,hydrogenation plateau of pristine Mg2Ni is 0.56 MPa while that of LaMg2Ni is 0.74 MPa at 603 K.According to the XRD result of LaMg2Ni alloy,after H/D process,LaMg2NitransformstoLahydride compound and Mg2Ni,compare with pristine Mg2Ni alloy,P-C-T curves suggest that the La hydride compound is helpful to the improvement of the hydrogen storage property for the Mg2Ni phase in the LaMg2Ni alloy at low temperature.

In order to study the thermodynamic property of LaMg2Ni alloy,P-C-T curves of LaMg2Ni alloy are plotted after the measurement of H/D process at different temperatures,the plateau pressure and temperature are plotted according to the Van′t Hoff equation(Eq.(3)).The Van′t Hoff plot for the hydrogenated LaMg2Ni alloy is shown in Fig.9.The enthalpy and entropy for the hydriding Mg2Ni in the LaMg2Ni alloy are calculated to be-53.02 kJ·mol-1,-84.96 J·K-1·mol-1(H2).Its hydride is less stable than pristine Mg2Ni alloy(-64.50 kJ·mol-1and-123.10 J·K-1·mol-1(H2),which shows that La hydride compound is beneficial to reducing the enthalpy and entropy for the hydriding Mg2Ni in the LaMg2Ni alloy.K⊖is the equi-librium constant,while=1/PHhydriding process.

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Effect of La hydride Compound on Hydriding Process of Mg2Ni Phase in LaMg2Ni Alloy

PEI Li-Chao1,2HAN Shu-Min＊,1,2ZHU Xi-Lin2LIU Bao-Zhong1ZHAO Xin2HU Lin2
(1State Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao,Hebei 066004,China)
(2College of Environmental and Chemical Engineering,Yanshan University,Qinhuangdao,Hebei 066004,China)

LaMg2Ni and Mg2Ni alloys were prepared by inductive melting under Ar atmosphere.X-ray diffraction(XRD)shows that during hydrogenation,LaMg2Ni alloy decomposes to LaH3phase and Mg2NiH4phase,in dehydriding process this alloy releases H2and LaH3phase changes to La3H7phase.Compared with Mg2Ni alloy,on account of the existence of La hydride compound and phase transition from La3H7phase to LaH3phase in hydriding process,LaMg2Ni alloy shows better hydriding kinetics.It is within 280 s for LaMg2Ni alloy to reach 90%of the maximum hydrogen absorption capacity,while it needs 1 200 s for pristine Mg2Ni alloy to do so.The rate constant of LaMg2Ni alloy is larger than that of Mg2Ni alloy at the same temperature.La hydride compound is beneficial not only to the enhancement of hydriding kinetics but also to the improvement of the thermodynamic properties.The enthalpy and entropy for the hydriding Mg2Ni in the LaMg2Ni alloy are-53.02 kJ·mol-1,84.96 J·K-1·mol-1(H2),respectively.Mg2NiH4in LaMg2Ni alloy is less stable than pristine Mg2Ni alloy (-64.50 kJ·mol-1and-123.10 J·K-1·mol-1(H2)).Pressure-Composition-Temperature (P-C-T)measurement results show that the hydrogen storage capacity of LaMg2Ni alloy is about 1.95wt%and is kept stable from 603 K to 523 K,while the hydrogen storage capacity of pristine Mg2Ni alloy declines distinctly form 4.09wt%to 3.13wt%with the reductionof temperature from 603 K to 523 K.The hydrogen storage capacity of pristine Mg2Ni alloy at lower temperature(523 K)is only 76.5%when compared to that at 603 K,suggesting that La hydride compound could improve the hydriding/dehydriding properties of Mg2Ni alloy at low temperature.

hydrogen storage alloy;phase transition;La hydride compound;hydriding kinetic

TG139+7

A

1001-4861(2012)07-1489-06

2011-10-07。收修改稿日期：2012-03-29。

国家自然科学基金(No.50971112,51001043)；国家863计划课题No.2007AA05Z117；河北省自然科学基金(No.E201001170)资助项目。

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